The present invention relates to machinery for corrugating tubes and more particularly to the corrugating head of such machines.
The use of corrugated tubes has become ever more prevalent in the field of communication and electrical transmission. Corrugated tubes are efficient devices for transmitting various information and/or electrical currents. Several different methods and apparatuses have been developed for manufacturing such tubes.
The present invention relates to a method and apparatus for corrugating tubes and more particularly for forming corrugations in the continuous production of tubing of the type used in high-frequency cables, wave guides and the like.
A number of methods and types of apparatus have been devised to produce corrugations in metallic (including metal-like) tubes. Corrugation is typically applied when there is a particular need for flexibility. There are two general classes of corrugated tubing, helical and annular. In helical tubing the tube is corrugated in a continuous helix along its length. In annular tubing the corrugation takes the form of individual corrugation annuli. Annular cables are typically used in outdoor applications where water migration is of concern. With helical cables the continuous form of the corrugated helix can permit water which enters through a hole to migrate along the helix through a length of the tube. The independent corrugations or annuli of an annular cable limit such migration.
Smoothness and uniformity of corrugation formation is found particularly critical in the case of tubing for high-frequency radiation transmission, such as coaxial cable and wave guides, where attenuation and compliance with mechanical specifications, such as military specification ML-C-28830C, is necessary. Corrugation imperfections and pitch variations too small to be readily observed have been found to produce highly undesirable reflections or standing waves.
Quality deficiencies can result from mechanical vibration, which may arise from various sources within the equipment. Bearings, sheaves, gearboxes, belts and pulleys can all be sources of such vibrations. The vibrations, while often small in magnitude, are typically periodic and can cause small inconsistencies in the formation of the corrugations, which in turn can create variations in the electrical response of the cable or tubing over the frequency spectrum of interest, and particularly variation in the voltage standing wave ratio (VSWR) of the cable. It is desirable that the VSWR be as low as possible, 1.00 being indicative of no reflection losses, with typical values of 1.05-1.15 being desired. The rotation of the typical corrugation head of a corrugating apparatus having several gears is a prime source for such vibrations, and is a significant area of concern in producing cables and wave guides of low and consistent VSWR.
U.S. Pat. No. 3,780,556 is illustrative of known systems for producing annularly corrugated tubes. A corrugating head has one or more gear-like corrugating wheels, each mounted both for free rotation about an axis transverse to the main, longitudinal axis of the tube to be corrugated and for orbital rotation about the longitudinal axis of the tube. The tube is drawn along the longitudinal axis through the corrugating head, while the corrugating wheels are orbited about the tube. The gear wheel teeth emboss annular corrugations in the tube, the individual gear teeth of the wheels aligning with the portion of the corrugation generated by the other gear wheels as a result of the free-wheeling nature of the gears. The general shape of the gears, as well as their orbital rotation rate and the longitudinal speed of travel of the tube through the head, are determined and applied as known in the art.
Although corrugating machines incorporating the foregoing technology are generally capable of producing annularly corrugated tubing in an efficient manner, due to the high tolerances required, particularly in connection with high the production of high frequency RF cables, it has heretofore been difficult to produce long runs of such cable without defects and/or inconsistencies which affect their performance.
It is accordingly a purpose of the present invention to provide a method and apparatus for tube corrugating, and particularly for a corrugating head of the multiple gear type, which exhibits improved performance and consistency of results.
Yet another purpose of the present invention is to provide a tube corrugating method and apparatus which allows increased lengths of annularly corrugated tube to be generated with greater precision and uniformity than has heretofore been produced.
In accordance with the foregoing and other purposes and benefits, the method of the present invention, and a tube corrugating apparatus constructed in accordance with the present invention, incorporate a multiple gear corrugating head in which each gear has a different number of gear teeth. Each gear has the teeth at the same pitch, such that each gear is thus of a different diameter. The number of teeth on each gear is chosen to minimize the constructive reinforcement of harmful vibrations which arise from gear rotation, such that the effects of such vibrations transmitted to the tube passing through the corrugating head and which manifests itself as tube inconsistencies, are minimized. In particular, the use of gears each having a different prime number results in minimization of additive vibration effects. The gear head may preferably be utilized in connection with a hollow shaft motor to further limit and minimize such vibrational effects.